The biggest headache I had when I used a backup MX was avoiding
backscatter. So I tweaked my milter on the primary to always accept mail
from the backup and never reject/bounce it. If necessary, silently drop
spam.
Alas, secondaries tend to be targets for spammers, on the assumption
they get less attention by admins and have weaker spam rules. If you run
SpamAssassin, you could add a small extra score for mail coming through
the secondary when the primary is known to be up.
https://en.wikipedia.org/wiki/Backscatter_(email)
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